This invention relates to protection switching for radio communication systems and, more particularly, it relates to switching between digital radio channels in an error free manner to insure system reliability.
Due to the occurrence in radio communication systems of frequency selective fading and possible equipment malfunctions, protection switching between a regular channel hampered by a transmission impairment and a standby, or spare, channel is known and used in the art. Each channel signal in such radio systems encounters a transmission delay that includes a constant portion whose value is pecullar to each channel and a time varying portion, because of the variability in atmospheric propagation introduced at each specific channel frequency.
In digital radio communication systems, this variable, or dynamic delay alone renders protection switching between a single standby channel, also known as a protection channel, and any one of a number of regular channels (referred to as 1XN) a difficult task. The precise timing attendant to digital signals is a further requirement such that 1XN protection systems for digital radio communications have heretofore been generally unavailable.
One type of conventional protection switching arrangement for digital transmission systems is a limited one-by-one (1X1) arrangement wherein a standby protection channel may only be substituted for a designated one of a number of regular channels. Such a protection provision is undesirable in terms of equipment duplication and congestion of the frequency spectrum at the radio frequencies.
The difficulty of this task has been so great that conventional protection switching systems even for one-by-one arrangements have principally been directed to maintaining framing on the digital bit screen rather than attempting the complete elimination of bit errors. Since a frame in a typical transmission format includes a predesignated large number of bits, the loss of framing will produce thousands of bit errors. However, the ultimate objective of any protection switching arrangement should be to provide error free operation when switching to a spare channel since only this is consistent with the intended purpose of offering such a feature.
Because the number of digital bit errors generally increases exponentially when the transmitted information signal experiences fading in a few decibels of attenuation, bit errors tend to grow rapidly. It would therefore be highly desirable for channel protection switching to occur when a transmission impairment is first detected with reliability to provide the best possible error performance by a digital radio transmission system. A further advantage to error free protection switching is the relaxation of constraints on switching for maintenance purposes since the error penalty of conventional protection switching arrangements restricts such maintenance switching to situations of absolute necessity.
A further requirement is occasioned by desynchronization arrangements used in current digital radio systems. The desynchronizers are used to provide the data signal and a recovered clock signal in the line receivers of such systems. Unfortunately, a pulse stuffing technique used in the desynchronizers produces timing jitter. This timing jitter is higher in frequency than the time varying portion of the variable delay mentioned previously. As a result, this places a further demand on the error free switching capability of a protection switching arrangement in a digital radio communication system.